Add and use and unlikely-to-be-eliminated memwipe()

Apparently some compilers like to eliminate memset() operations on
data that's about to go out-of-scope.  I've gone with the safest
possible replacement, which might be a bit slow.  I don't think this
is critical path in any way that will affect performance, but if it
is, we can work on that in 0.2.4.

Fixes bug 7352.

src/or/routerparse.c

@@ -4596,7 +4596,7 @@ tor_version_parse(const char *s, tor_version_t *out)
     if (close_paren-cp > HEX_DIGEST_LEN)
       return -1;
     hexlen = (int)(close_paren-cp);
-    memset(digest, 0, sizeof(digest));
+    memwipe(digest, 0, sizeof(digest));
     if ( hexlen == 0 || (hexlen % 2) == 1)
       return -1;
     if (base16_decode(digest, hexlen/2, cp, hexlen))